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Update Abstracts (Current
Projects)
Opportunities for Climate and
Air Quality Research Collaboration at ARM Sites The Atmospheric Radiation Measurement (ARM) Program was created in 1989 with funding from the U.S. Department of Energy (DOE) to help resolve scientific uncertainties about global climate change with a specific focus on improving the performance of general circulation models (GCMs) used for climate research and prediction. The ARM program has supported the development of several highly instrumented ground stations for studying cloud formation processes and their influence on radiative transfer and for measuring other parameters that determine the radiative properties of the atmosphere. Scientists collect and analyze data obtained over extended periods of time to study the effects and interactions of sunlight, radiant energy, aerosols, and clouds on temperatures, weather, and climate. In addition, several intensive operational periods (generally aircraft based) are conducted every year to focus on process-specific research. A mobile or transportable facility is being planned that can be used for shorter-term studies (time periods on the order of 1 year). A User Facility is being developed in association with the program to facilitate collaborative efforts at ARM field sites with other research groups focusing on related issues such as air quality. Carbonaceous aerosols have been identified as being of particular interest for both climate and air quality related purposes. A brief overview
of EPA/ORD ambient PM methods research program Continuous Characterization of Carbon
in Fine Particulate Matter As a part of the summer intensive studies of particulate matter (PM) in Rochester, NY and Philadelphia, PA, highly time resolved measurements of PM2.5 mass, organic carbon (OC), elemental carbon (EC), black carbon (BC), and particulate sulfate were made. Continuous, semi-continuous, and daily filter-based samples were collected in Rochester from June 3 to 18, 2002 and from July 1 to August3, 2003 in Philadelphia. In order to minimize the loss of semi-volatile compounds in fine PM and condensation of water, TEOM system (R&P) was operated at a fixed temperature 30 oC with a Sample Equilibration System dryer. A semi-continuous OC/EC analyzer (Sunset Lab) measured 2-hour averaged OC and EC mass concentrations of fine PM with an upstream OC denuder to prevent the positive artifacts of semi-volatile organic compounds. High resolution particulate sulfate concentrations were measured by a continuous sulfate analyzer (Harvard School of Public Health design). During the Rochester intensive program conducted between June 6 and 18, 2002, BC and UVPM were approximately same and highly correlated (r2 = 0.98, slope = 0.91). Thermal EC and optical EC were highly correlated and the thermal EC was about 11 % higher than the optical EC. BC was more correlated with optical EC (r2 = 0.78) than thermal EC (r2 = 0.72). However, BC was much higher than the optical EC. During the Philadelphia summer intensive PM program measured between July 10 and August 3, 2002, UVPM was approximately 18% higher than Rochester UVPM. The correlation coefficient between optical EC and BC (r2 = 0.78) was consistent with the correlation coefficient of Rochester study. Also, BC was 30% higher than the optical EC indicating inclusion of light-absorbing OC in Aethalometer BC. On July 2, 2002, a Canadian forest fire started and severe smoke was transported to the monitoring site from July 6 to 9, 2002. During the measurement period, an abrupt increase of PM2.5 was observed on the afternoon of July 6. The highest PM2.5 mass concentration was greater than 160 _g m-3 late in the evening of July 7. Peaks in the measured semi-continuous thermal and optical EC were observed on same day. The thermal EC was 70% higher than the optical BC. In addition, UVPM was much higher than BC during the Canadian forest fire. It strongly suggests that there was substantial amounts of aromatic organic compounds in PM2.5 during the forest fire. Also, the difference between optical EC, thermal EC, BC, and UVPM indicate that these measurement methods are strongly dependent on the chemical composition of PM2.5. During July 7 to 8, sulfate aerosol concentration decreased to only 5.5 % of PM2.5, whereas optical / thermal OC and EC increased to over 100 % of the mass measured with the 30C TEOM indicating the loss of semivolatiles in the mass measurements. 1 Sponsored in part by EPA PM Center grant R827453. OC/EC Considerations in Phoenix and
Downwind Class I Areas For metropolitan Phoenix, Arizona, recently constructed emission inventories of speciated fine particulates, based in part on the OC/EC splits determined for gasoline and diesel vehicle exhaust in the late 1990's in several national experimental programs, have an OC/EC ratio of 0.4. Extensive ambient measurements made at three sites in the fall and winter of the mid 1990's have an OC/EC ratio four times as great – 1.7. Positive sampling artifacts from the adsorption of gaseous hydrocarbons onto the quartz filter cannot account for this discrepancy. Nor can the formation of secondary organic aerosols, which in the marginally photochemically reactive winter atmosphere, would never account for the difference. This mis-match between the composition of the inventory and the aerosol has introduced considerable and unwelcome uncertainty in the relationship between the emissions of carboneous fine particles and their resultant concentrations and consequent light extinction. Furthermore, Phoenix lies 30 miles upwind of three Class I wilderness areas, and transport of the urban plume into this region is of concern in complying with the Regional Haze regulations. The Arizona Department of Environmental Quality is sponsoring an ongoing urban organic tracer experiment, and is planning a wilderness area carbonaceous particulates experiment, both with Arizona State University researchers, to better determine the sources and transport of OC/EC. This presentation outlines this experimental work. Measuring Elemental Carbon Absorption
Using a Dual Thermal Optical Reflectance/Transmittance Analyzer An innovative thermal optical carbon analyzer that features a flexibly programmable heating unit and concurrent monitoring of laser reflectance (TOR) and transmittance (TOT) is used to analyze OC/EC in ambient samples from Hong Kong and United States under various thermal protocols (STN, IMPROVE, etc). With reflectance and transmittance measurements, this technique allows a more accurate estimate of aerosol absorption and scattering on the filter. Absorbance at the beginning of the thermal analysis is strongly correlated to EC abundance determined by STN protocol at r2 > 0.97. This confirms that elemental carbon is the single dominant light-absorbing species and also yields a mass absorption efficiency between ~ 12 m2 g-1 (TOR EC) and ~ 15 m2 g-1 (TOT EC), close to previously estimated values of 5 – 15 m2 g-1 (EC). The increase of aerosol scattering/absorption ratio during the first part of thermal analysis suggests that pyrolyzed OC could have a higher scattering albedo than native EC; this explains why TOR EC is often higher than TOT EC. Combining absorption and carbon measurement at the second part of thermal analysis (after the complete OC depletion) yields another close estimate of EC absorption efficiency and helps determine the best laser split point. Charring Minimization in Thermal Analysis
of Aerosol Carbon Our previous work has shown that charring of organic carbon during thermal analysis is largely responsible for the uncertainties of ECOC measurements. Minimizing charring therefore improves the accuracy of setting the ECOC split in thermal/optical methods. We have identified two means to reduce charring, i.e. addition of a low temperature oxidation step prior to aggressive heating in the helium atmosphere and lengthening the residence time at each temperature step to allow maximum OC evolution. Preliminary results will be presented in the workshop. Uncertainties in Optical Charring Correction Schemes The correct speciation of OC and EC in thermal/optical methods depends on one of the following two assumptions: (1) PEC evolves before native EC evolves in the analysis, or (2) PEC and native EC have the same apparent light absorption coefficient (s) at the monitoring light wavelength. Neither of these assumptions has actually ever been checked or tested. The first assumption is invalidated by the observation that the combustion of PEC overlaps that of native EC despite multiple step-wise combustion at temperatures ranging from 575 to 910oC. An examination of s versus EC evolution indicates that the s values of PEC and EC are not the same in most cases and the s value of PEC is not constant during a single thermal analysis. The second assumption is thus invalid as well. The measured EC concentrations can either overestimate or underestimate the true native EC concentrations depending on the relative magnitude of the s values of the PEC and native EC at the point where the instrument sets the EC/OC split line. Both over- and underestimation have been observed in real aerosol samples. The unequal s values of PEC and EC also explain that different temperature programs, when employed to analyze the same filter samples, systematically yield different EC and OC concentrations.
Reconciling Carbon Measurements
Between the EPA Speciation Trends and IMPROVE Networks Implications of using OC/EC to estimate fire and soa contributions to carbbonaceous material William Malm, National Park Service, CIRA Joseph Conny, Surface and Microanalysis Science Division, National Institute of Standards and Technology (Mr. Conney has prepared two abstracts listed as follows) How Does The Thermal-Optical Transmission Method Behave Optically? A Focus On The Apparent Specific Absorption Cross Section
A Response Surface Approach to Optimizing Thermal-Optical Methods [1]
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